Broadband rotary transformer



ct. I7, 1967 c. STROMSWOLD 3,348,181 BROADBAND ROTARY TRANSFORMER Filed April 26, 1965 INVENTOR CHESTER STROMSWOLE B ATTORNE United States Patent 3,348,181 BROADBAND ROTARY TRANSFORMER Chester Stromswold, Nashua, N.H., assignor to Sanders Associates, Inc, Nashua, N.H., a corporation of Delaware.

Filed Apr. 26, 1965, Ser. No. 450,980 7 (Ilaims. (Cl. 336-120) ABSTRACT OF THE DISCLOSURE This invention relates to a rotary coupling for transferring electrical energy between two members arranged for rotation relative to each other. More particularly, it relates to a broadband rotary transformer in which the primary is the outer conductor of a substantially coaxial system having the transformer secondary as its inner conductor. This configuration greatly reduces the leakage inductance, thereby materially extending the bandwidth limit of operation.

Rotary couplings are required for the transmission of electrical energy between fixed base'members and rotating elements such as radar antennas or the like. At sufliciently high frequencies a coupling of this type may take the form of a pair of'abutting circular waveguide sections with a rotary joint between them. However, where the energy to be transferred through the coupling includes relatively low frequency components a waveguide is impractical because of the size required. Therefore, two other types of coupling have been used. One of these is a conventional slip ring arrangement comprising rings on the rotating member contacted by stationary brushes. This device is characterized by undesirable electrical noise as well as brush wear, both of which detract from serviceability, particularly where long periods of unattended operation are contemplated.

Rotary transformers have also been used prior to the present invention to transfer electrical energy between two relatively rotating parts.' While these transformers avoid the problems encountered with slip rings, they have exhibited another characteristic of their own. Specifically, the leakage inductance severely limits the attainable bandwidth. 7

Accordingly, it is an object of the present invention to provide an improved rotary coupling for use in transfer ring electrical energy from one member to another member arranged for rotary motion relative thereto.

Another object of the invention is to provide a rotary coupling of the above type having a low-loss characteristic.

A further object of the invention is to provide a coupling of the above type having a uniformly low attenuation over a broad range of frequencies.

Yet another object of the invention is to provide a coupling of the above type characterized by minimal generation of electrical noise.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is an axial section of ing the invention,

FIG. 2 is an axial section of the primary section of the coupling,

FIG. 3 is a plan View of the face of the primary section of the coupling,

FIG. 4 is an axial section of the secondary section of the coupling, and

FIG. 5 is a plan view of the section.

In general, the present invention is embodied in a rotary transformer whose primary and secondary are in the form of loops having a common axis and mounted for rotation relative to each other about this axis. The primary is constructed as a circular tube having a longitudinal slot therein; the secondary is disposed within the tube, with the secondary leads extending from the slot. Thus, the primary and secondary have a combined structure similar to a coaxial cable.

It is well known that the entire magnetic field developed by current in the outer conductor of a coaxial cable encircles the inner conductor of the cable. Thus, a transformer using the outer conductor as a primary and the inner conductor as a secondary has no leakage flux. In the present case, the slot in the primary loop, required for access to the inner secondary loop, prevents complete elimination of the leakage flux. However, there is a very substantial reduction as compared with prior rotary transformers and this reduction is sufiicient for operation over a broad range of frequencies, extending from 30 to 1,000 megacycles in one unit constructed according to the in vention, without undue attenuation.

As shown in FIG. 1 a rotary transformer embodying the invention includes a primary section 10 and a secondary section 12 mounted for relative rotation about an axis 14. .In the illustrated embodiment the secondary sect'ion 12 is supported at the end of a stationary tube 16 and the primary section 10 is supported by set screws 17 in an outer tube 18 suitably coupled to a drive system (not shown) capable of rotating the tube 18 about the axis 14;

As shown in FIGS. 1, 2 and 3, the primary section 10 includes a base 20 provided with an annular recess 22 opening toward the secondary section 12 and centered on the axis 14. The recess 22 has a metallic lining which serves as a primary loop 24, i.e. the primary winding of the transformrer. Primary .loop 24, which is in the form of a circular tube, has an aperture or slot 38 facing base 28 of the secondary. As shown in FIG. 3 the loop 24 is interrupted to provide ends 24a and 24b connected respectively to the outer and inner conductors of a coaxial cable 26. The cable 26 extends upwardly through an aperture 27 (FIG. 2) in the base 20 for connection to a source of electrical signals (not shown).

As shown in FIGS. 1, 4 and 5, the secondary section 12 includes a circular base 28 to which is secured a dielectric ring 30. The ring 30, which fits within the primary loop 24, carries on its upper surface a metallic annulus serving as a secondary loop 32. As shown in FIG. 5, the loop 32 is interrupted, providing ends 32a and 32b to which the outer and inner conductors of a coaxial cable 34 are connected. The cable 34 extends downwardly through an aperture 36 in the base 28 for connection to a device (not shown) receiving the signals passing through the transformer.

In order to avoid the effects of friction, particularly at high rotational velocities, the primary and secondary sections 10 and 12 are preferably spaced from each other. Thus, the dielectric ring 30 does not touch the primary a rotary coupling embodyface of the secondary loop 24, and there is a small gap, of the order of 0.005 inch between the bases and 28.

Since the primary loop 24 is in the form of a tube, except for the circular aperture or slot 38 thereof facing the base 28 (FIG. 1), almost all of the magnetic field associated with current conducted by this loop is outside the loop. Thus, almost the entire magnetic field links the secondary loop 32. Even the portion of this field which is within the interior of the loop 24 because of fringing effects largely links the secondary loop 32 because of the distance from the aperture 38 to the loop 32. Accordingly, the leakage inductance of the transformer is very small, and this materially extends the high frequency range of the coupling.

Also, the bases 20 and 28 are preferably made of ferrite so as to provide a low reluctance path for magnetic fields in the desired regions surrounding the primary loop 24. This is particularly helpful in maintaining a high coupling coefficient at the lower frequency end of the operating range.

The coupling coefficient of the transformer is relatively independent of the angular position of the primary with respect to the secondary as long as the circumference of the primary and secondary loops is substantially shorter than a wavelength at the frequencies of the signals to be transferred by the coupling.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained, and since certain changes may be made in carrying out the above method and in the construction set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

For example, the transformer may be provided with a step-up ratio by providing the secondary loop 32 with a plurality of turns instead of the single turn in the illustrated embodiment of the invention, Also, the slot 38 need not face in the axial direction as shown in the drawings. It may face radially outward or inward from the primary loop 24, although the illustrated configuration provides for easier fabrication of the transformer.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be 'said to fall therebetween.

What is claimed is:

1. A rotary transformer comprising (A) a substantially circular single turn primary conductor having (1) a tubular cross-section, and (2) a longitudinal slot,

(B) a substantially circular single turn secondary conductor (1) disposed within said primary conductor, and (2) extending longitudinally therein,

(C) means for mounting said primary and secondary conductors for continuous relative rotation about the axis of the circle defined by said primary conductor.

2. The combination defined in claim 1 including electrical means extending from said secondary conductor through said slot.

3. The combination defined in claim 1 including high permeability magnetic material surrounding said primary conductor.

4. A rotary transformer comprising (A) a single turn primary in the form of a tubular first conductor (1) whose axis describes a first circle, and 2) having a longitudinal slot therein, (B) a single turn secondary in the form of a second conductor (1) disposed within said primary conductor, and (2) describing a circle substantially concentric with said first circle,

(C) an electrical lead extending from said secondary conductor through said slot, and

(D) means mounting said primary and secondary for relative rotation about the center of said circles.

5. The combination defined in claim 4 including material having a high magnetic permeability substantially surrounding said primary.

6. A rotary transformer comprising (A) first and second base members of high permeability magnetic. material,

(B) means forming a circular recess in said first base member opening toward said second base member,

(C) a single turn primary in the form of a tubular first conductor (1) disposed in said recess, (2) having an axis describing a first circle, (3) having a longitudinal slot facing said second base member, (D) a single turn secondary in the form of a second conductor (1) mounted on said second base member, (2) whose axis describes a second circle substantially concentric with said first circle, (3) disposed within said first conductor, (E) a first lead conductor 1) connected to said secondary, and (2) extending from said slot,

(F) a second lead conductor connected to said primary,

and

(G) means mounting said first and second base members for relative rotation thereof about the center of said first and second circles.

7. The combination defined in claim 6 in which at least the portions of said first and second base members sur-' rounding said primary are comprised of high permeability magnetic material.

References Cited UNITED STATES PATENTS 428,618 5/1890 Lemp 336- 2,894,231 7/1959 Krasno 336-123 X FOREIGN PATENTS 515,865 1/1931 Germany.

LEWIS H. MYERS, Primary Examiner.

T. J. KOZMA. Assistant Examiner. 

1. A ROTARY TRANSFORMER COMPRISING (A) A SUBSTANTIALLY CIRCULAR SINGLE TURN PRIMARY CONDUCTOR HAVING (1) A TUBULAR CROSS-SECTION, AND (2) A LONGITUDINAL SLOT, (B) A SUBSTANTIALLY CIRCULAR SINGLE TURN SECONDARY CONDUCTOR (1) DISPOSED WITHIN SAID PRIMARY CONDUCTOR, AND (2) EXTENDING LONGITUDINAL THEREIN, (C) MEANS FOR MOUNTING SAID PRIMARY AND SECONDARY CONDUCTORS FOR CONTINUOUS RELATIVE ROTATION ABOUT THE AXIS OF THE CIRCLE DEFINED BY SAID PRIMARY CONDUCTOR. 